Given an encoded string s in the form k[encoded_string] (where k is a positive integer, possibly nested arbitrarily deep), decode it by repeating each bracketed substring k times. Recursively parsing this works, but finding each bracket's match by scanning forward wastes effort re-treading the string. A single-pass approach with two stacks avoids that entirely: track the string being built at the current nesting level, and whenever a new level opens ('['), push the pending repeat count and the string built so far, then start fresh. Whenever a level closes (']'), pop both back off and fold the finished level in — repeat it and prepend what came before. The stack always knows exactly where to resume, with no need to ever locate a matching bracket's position in advance.